This invention relates to a process for the manufacture of shaped structures having a high piezoelectric property out of a copolymer of vinylidene fluoride.
Heretofore, quartz, Rochelle salt, barium titanate ceramics and other inorganic piezoelectric materials have been known, and recently it has been ascertained that certain biological materials and synthetic high molecular weight compounds also possess a piezoelectric property. The biological tissue which has been ascertained to possess a piezoelectric property includes, e.g., bone, skin, blood vessel, trachea, muscle, hair, ivory, silk, bamboo and wood, and it is now believed that most proteins possess a piezoelectric property. A piezoelectric property is observable also on monoaxially stretched films of poly(.alpha.-benzyl glutamate), poly(.gamma.-methyl glutamate) and like synthetic polymers of single amino acids. Other than these instances, it has long been known that when a high molecular weight polymer film is subjected to the action of an electrostatic field of a high potential gradient at a relatively high temperature and then cooled in the electrostatic field to room temperature as to set it in a state where it is dielectrically polarized in the direction perpendicular to its both surfaces, there is formed a so-called electret. Although it was known that the electret possesses a piezoelectric property, it was extremely small.
Recently, with the development of high polymer electrets, determinations of piezoelectric properties of various electrets have been made, and it has been found that films of polyvinylidene fluoride (hereinafter polyvinylidene fluoride will be referred to as PVDF), polyvinyl fluoride, polyvinyl chloride and like polar high molecules possess, in particular, an ever-lasting piezoelectric property. Especially with PVDF, a high piezoelectric property has been observed and, for instance, a piezoelectric PVDF film having a piezoelectric modulus d31, taking the direction of monoaxial stretching of the film as the X-axis, of the order of 10.sup.-7 c.g.s.e.s.u., has been obtained by monoaxially stretching a PVDF film by a few times its original dimension at 120.degree. to 150.degree. C. and polarizing the stretched film in an electrostatic field of a potential gradient of about 300 KV/cm at 80.degree. to 90.degree. C.
High polymer polarized films having high piezoelectric moduli all were formed hitherto, on the basis of a monoaxially stretched film, and none was made from an unoriented film or molding. With respect to piezoelectric films formed of PVDF, a high piezoelectric modulus was found on a stretched and molecular oriented film.
Piezoelectric high molecular weight films in most cases exhibit a tensile piezoelectric property and, in this case, the piezoelectric property is indicated by a piezoelectric modulus d attributable to a dielectric polarization in a direction perpendicular to the surface of the film under a tensile stress.
With respect to piezoelectric high polymer films, taking the direction of monoaxial stretching as the X-axis, the direction perpendicular to the X-axis and parallel to the direction of the surface of the film as the Y-axis, and the direction perpendicular to the surface of the film as the Z-axis, to determine X-, Y- and Z-axes, and taking the piezoelectric modulus indicating a polarization in the direction of the Z-axis under a tensile stress in the direction of the X-axis as d.sub.31, and that under a tensile stress in the direction of the Y-axis as d.sub.32, the d.sub.31 and d.sub.32 in most cases differ from each other and, in case of a monoaxially stretched film of PVDF, the value of d.sub.31 reached about 10 times that of d.sub.32 or more. And, with respect to a monoaxially stretched film of poly(.gamma.-methyl glutamate), it is known that the piezoelectric modulus in the direction of the Z-axis takes a maximum value when the film is tensioned in the direction meeting both the X- and Y-axes at angles of 45.degree.. Thus, most of such stretched high polymer films are anisotropic in piezoelectric property. Expecially in the case of monoaxially stretched PVDF films, it is possible to obtain a polarized film having a piezoelectric modulus d.sub.31 of 10.sup.-6 c.g.s.e.s.u. or more at maximum and such an electret is of great value, but they have drawbacks, e.g., when used as a diaphragm in a loud speaker, the polarized film vibrates with extremely different amplitudes in the direction of its X-axis and of its Y-axis, namely, it cannot vibrate with uniformity, by virtue of the high anisotropy as mentioned above, and, so long as it is a monoaxially stretched film, there is a limit to its thickness and form.
On the other hand, unoriented PVDF films and PVDF moldings have no anisotropy with respect to piezoelectric property and thick sheets and pipes, spheres and other optional shapes are obtainable, but it is difficult to form a polarized film therefrom having a piezoelectric modulus exceeding 10.sup.-8 c.g.e.s.u.